Molecular and functional characterization of opsins in barfin flounder (Verasper moseri)

- We identified eight visual opsin molecules from barfin flounder.
- One of them is a pseudogenized green-sensitive opsin gene.
- Maximum absorption spectra of the functional opsins were determined.
- One of two blue-sensitive opsins changed into green-sensitive.
- Expression levels of the opsin genes changed through ontogeny.

Green light irradiation facilitates the somatic growth of barfin flounder (Verasper moseri). However, the V. moseri visual system, which may be associated with somatic growth by acting on the endocrine system upon exposure to this particular wavelength, remains largely unexplored. Herein, we characterized the visual opsin repertoire of V. moseri to understand the molecular basis underlying this effect. The five types of visual opsins that are found in vertebrates were cloned from RNA that was extracted from the eyes of V. moseri. Notably, V. moseri possessed one pseudogene (RH2-A) and two intact (RH2-B and RH2-C) copies of “green-sensitive” opsin genes. The wavelengths of maximum absorption spectra (λmax) for each of the reconstituted photopigments were 552 nm for “red-sensitive” LWS, 506 nm for RH2-B, 490 nm for RH2-C, 482 nm and 416 nm for “blue-sensitive” SWS2A and SWS2B, respectively, 367 nm for “ultraviolet-sensitive” SWS1, and 494 nm for “dim-light sensitive rhodopsin” RH1. The λmax of SWS2A was longer than that of any other reported vertebrate SWS2 opsin. By measuring the expression level of these opsin genes with quantitative RT-PCR in 3-, 15-, and 27-month-old fish, we found that RH2-B and SWS2A were expressed at a constant level, whereas the expression of LWS, RH2-C, SWS2B, and SWS1 opsin genes decreased, and that of RH1 increased with age. Barfin flounders inhabit inshore waters at a young age and expand their habitat to deep sea areas as they age, and green light is relatively abundant in deep water compared to the lights of other wavelengths in shallow water. Our results indicate that gene repertoire and expression profile of the opsin genes of barfin flounder are adaptive to their habitat shift that occurs during development, with some opsins acquiring a distinct λmax.